The present invention relates to optical fiber communication systems and more particularly to the communication of digital signals over long distances using optical sources that have optical frequency chirp.
The communication of signals over optical fibers has many advantages over electrical transmission using wires. Optical fibers currently in production can support virtually unlimited bandwidth with low attenuation. In optical communication systems, a light source, e.g., from a laser or light emitting diode (LED), is modulated by an information signal that is carried by the light via the optical fiber. The light source can be directly modulated or externally modulated by the information signal. The modulation of the carrier can comprise frequency modulation or amplitude modulation.
Various known optical sources, such as semiconductor lasers, exhibit optical frequency chirp when directly modulated by an information signal. Chirp is defined as a rapid change (as opposed to a long-term drift) of the emission wavelength of an optical source, and is often observed in pulsed operation of a source.
Long distance digital and frequency modulation systems that use optical sources having optical frequency chirp suffer degradation due to optical dispersion in the transmission fiber. In a digital system, dispersion causes the digital pulses to spread. As a result, the pulses can overlap and interfere with each other, limiting data transmission speed. Since the pulses stretch out a certain amount each kilometer of fiber, pulse dispersion increases linearly with the distance traveled. Dispersion is particularly troublesome in long distance transmission, where it causes a severe degradation in the system bit error rate.
A typical optical source for communicating signals over long distances is the direct feedback (DFB) laser diode. One technique that has been used to reduce the chirp of a directly modulated DFB laser diode is to use a lower modulation depth. This method, however, also leads to lower extinction ratio, which makes it more difficult to discriminate between the transmitted digital levels. More particularly, in the optical communication of digital signals, the light intensity is either "on" (e.g., a digital 1) or "off" (e.g., a digital 0). The extinction ratio is defined as the ratio of the intensity when the light source is "on" to the intensity when the light source is "off." Ideally, the extinction ratio would be infinity. However, this is impossible to achieve in a practical system, where an extinction ratio on the order of 20 dB is considered to be very good.
DFB lasers that have been proposed for use in communication systems where a receiver directly detects light intensity have both amplitude modulated (AM) and frequency modulated (FM) components. The presence of FM components degrades system performance. More particularly, the greater the FM modulation index (i.e., the greater the difference between the wavelength associated with a digital 1 and the wavelength associated with a digital 0), the more dispersion will occur over the transmission fiber in the time domain. Thus, dispersion has been a serious problem in systems where it is desired to directly detect digital signals communicated over an optical communication path by a light source exhibiting optical frequency chirp.
It has been proposed to use an unbalanced Mach-Zehnder interferometer as an optical filter in such systems to pass only the higher intensity modulated light. This has been found to increase the extinction ratio. See, e.g., E. G. Bryant, et al., "Two-Way Transmission at 2.488 Gbit/s Over a 132 km Operational Submerged Cable Using Erbium-Doped Fibre Power Amplifiers," Electronics Letters, Vol. 26, No. 17, Aug. 1990, pp. 1355-1357 and e.g., Bryant, et al., "Unrepeatered 2.4 Gbit/s Transmission Experiment Over 250 km of Step-Index Fibre Using Erbium Power Amplifier," Electronics Letters, Vol 26, No. 8, April 1990, pp. 528-530. The use of an unbalanced Mach-Zehnder interferometer in the system described in the referenced publication adds expense to the system and it may be difficult to maintain the alignment of the Mach-Zehnder optical filter with the optical frequency of the laser.
It would be advantageous to provide an alternative method for increasing the extinction ratio in a long distance digital system that uses an optical source having optical frequency chirp. Such a scheme should enjoy relatively low cost and be easy to implement. The present invention provides apparatus for use in transmitting digital data over an optical communication path enjoying the aforementioned advantages.